UPPER_CONSTRAINTS_FILE is deprecated and TOX_CONSTRAINTS_FILE is the new environment variable name that replaces it [1]. This allows to use upper-constraints file as more readable way instead of UPPER_CONSTRAINTS_FILE=<lower-constraints file>. [1] https://zuul-ci.org/docs/zuul-jobs/python-roles.html#rolevar-tox.tox_constraints_file [2] https://review.opendev.org/#/c/722814/ Change-Id: I4fdd57d6db4c6e092274ef9d5c34a7e5a2bf8222changes/86/761386/4